Narcissism defines the narcissist's waking moments and his nocturnal dreams. It is all-pervasive. Everything the narcissist does is motivated by it. Everything he avoids is its result. Every utterance, decision, his very body language - are all manifestations of narcissism. It is rather like being abducted by an alien and ruthlessly indoctrinated ever since. The alien is the narcissist's False Self - a defence mechanism constructed in order to shield his True Self from hurt and inevitable abandonment.
Cognitive understanding of the disorder does not constitute a transforming INSIGHT. In other words, it has no emotional correlate. The narcissist does not INTERNALIZE what he understands and learns about his disorder. This new gained knowledge does not become a motivating part of the narcissist. It remains an inert and indifferent piece of knowledge, with minor influence on the narcissist's psyche.
Sometimes, when the narcissist first learns about the Narcissistic Personality Disorder (NPD), he really believes he could change (usually, following a period of violent rejection of the "charges" against him). He fervently wants to. This is especially true when his whole world is in shambles. Time in prison, a divorce, a bankruptcy, a death of a major source of narcissistic supply - are all transforming life crises. The narcissist admits to a problem only when abandoned, destitute, and devastated. He feels that he doesn't want any more of this. He wants to change. And there often are signs that he IS changing. And then it fades. He reverts to old form. The "progress" he had made evaporates virtually overnight. Many narcissists report the same process of progression followed by recidivist remission and many therapists refuse to treat narcissists because of the Sisyphean frustration involved.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)